Mobile communication systems such as cellular and personal communication systems (PCS) have become ubiquitous. Such systems commonly include base station hardware, which may be commonly referred to as infrastructure, that exchanges information with mobile stations such as mobile telephones or the like. As will be appreciated, a particular base station hardware installation provides communication service coverage for the geographical area in which the base station hardware installation is located. Accordingly, multiple base station hardware installations are required to provide communication services to broad geographical areas. Each base station is capable of handling a finite number of calls at one time.
The convenience and relatively low cost of mobile stations and their attendant services has lead to a high market penetration, resulting in numerous geographical areas in which communications traffic is extremely dense. As communication traffic density increases, it becomes necessary for more base station hardware installations to be distributed throughout a given geographical area. The increased geographical density in base station installations can lead to problems such as, for example, increased interference.
Communication system technology may be roughly lumped into several categories: time-division multiple access (TDMA), frequency-division multiple access (FDMA) and code-division multiple access (CDMA), of which TDMA and CDMA are becoming the most popular. As the name implies, a TDMA system operates on a time-multiplexed scheme in which a base station listens only to one mobile station during a particular time slot. One example of TDMA technology is the iDEN (integrated dispatch enhanced network) system designed and provided by Motorola, Inc. of Schaumburg, Ill.
Due to of the great number of base station sites and a finite frequency band allocated to TDMA systems, base stations located in geographically disparate areas from one another may use the same frequencies to communicate with mobile stations. This concept is called frequency re-use. As demand for TDMA mobile stations and service density increased, base stations geographically close to one another began to reuse frequencies, which lead to an increased probability of a mobile station in communication with a first base station on a frequency (e.g., f1) interfering with the ability of a second base station to receive a desired signal at that same frequency (e.g., f1). This interference situation is referred to as co-channel interference because the interference is located on the same channel (i.e., is at the same frequency) as the desired signal.
As will be readily appreciated, TDMA mobile stations typically operate on battery power and battery life is a major concern. To conserve battery life, a mobile station may turn off its transmitter when the mobile station is not sending information to a base station. For example, if a mobile station user is only listening to information on his or her mobile station, the mobile station may disable its transmitter. The situation in which the mobile station disables its transmitter is commonly referred to as DTx (discontinuous transmit).
When a mobile station enters a DTx mode of operation, the base station with which that DTx mode mobile station was communicating is susceptible to a particularly deleterious effect of co-channel interference that is commonly referred to as cross talk. Cross talk can result when desired and undesired mobile stations are within reception range of a base station, but the desired mobile station is closer to the base station and, therefore, provides sufficient transmit power to overpower the undesired mobile station so that the base station only receives information from the desired mobile station. If the desired mobile station enters the DTx mode, the transmit power provided by the desired mobile station drops to a level below that of the undesired mobile station. Accordingly, upon DTx of the desired mobile station, the base station may receive undesired audio transmissions from the undesired mobile station. The undesired audio transmissions may be processed by the base station and, therefore, unintended audio may be coupled to the telephone of the person who was previously communicating with the user of the desired mobile station. The reception of the undesired mobile station transmission by the base station is known as cross talk.
In an attempt to eliminate or at least reduce the probability of cross talk in the iDEN system, one of sixteen different color codes, or designators, were assigned to base stations and those color codes were used by mobile stations when communicating with the base stations. The addition of color designators provided an auxiliary manner in which base stations could distinguish a co-channel interferer (i.e., a potential source of cross talk) from a desired transmission because only the desired transmission would have the proper color designator of the base station embedded therein. The sixteen color designators were assigned to base stations so that geographically proximate base stations that may reuse a frequency had different color designators. When receiving communications, each base station would look for its own color designator in the received signal. If the proper color designator were present in the signal, the base station would process the signal. Alternatively, if the proper color designator were not present, the base station would disregard the received signal as cross talk.
An exemplary illustration of a color code system 5 is provided in FIG. 1. The color code system 5 includes a number of base stations 10–14, which are referred to as the blue base station 10, the green base station 12 and the red base station 14. Each base station 10–14 provides a cell of communication coverage, referred to with reference numerals 16–20. In general, mobile stations communicate with the base station responsible for maintaining the cell in which the mobile station is located. For example, a mobile station 22 communicates with the blue base station 10 and mobile stations 24 and 26 communicate with the green base station 12. Additionally, a mobile station 28 communicates with the red base station 14.
As shown generally in FIG. 1, each mobile station 22–28 communicates with its respective base station 10–14 using a frequency (e.g., f1–f4). In the transmit path from the mobile station 22–28 to the base station 10–14, the mobile units also communicate the color of the base station 10–14 with which they are communicating. For example, the mobile station 22 receives communication from the blue base station 10 at frequency f2, but transmits information to the blue base station 10 on frequency f1 and includes in the information transmitted on frequency f1 the color blue. Accordingly, FIG. 1 reflects communication from the mobile unit 22 to the blue base station 10 with the designator f1B, representing frequency one, blue base station.
The mobile unit 24 also uses f1 to communicate with the green base station 12, but it also includes the color green (G) in its transmissions. Likewise, the mobile unit 26 communicates with the green base station 12 on frequency f3 with the green designator (G). Further, the mobile station 28 communicates with the red base station 14 using f3, but includes the red designator (R).
The use of the color designators prevents the base stations 10–14 from inadvertently processing cross talk signals received from a mobile station transmitting on the same frequency as a desired mobile station, but whose signals are not intended for the base station that inadvertently receives the signals. For example, as shown in FIG. 1, the green base station 12 may be exposed to a signal from the mobile station 22, which transmits at frequency f1. The use of color designators enables the green base station 12 to receive the signal from the mobile station 22, to determine that such a signal is not intended to be received by the green base station 12 and to discard any cross talk information received from the mobile station 22. Similarly, the green base station 12 may disregard co-channel information received from the mobile station 28, because such information is not intended for the green base station 12 and, in fact, may be the source of cross talk in the system.
Referring to FIG. 2, an exemplary slot 40 that may, for example, be transmitted by a mobile unit in the iDEN system includes a number of symbols, four exemplary types of which are referred to with reference numerals 42–48. The slot 40 may be 15 milliseconds (ms) in length and may include four subchannels 50–56 in which the symbols 42–48 may be transmitted.
The symbol 42 is representative of a training waveform that is transmitted by the mobile station upon power changes, frequency changes or if the mobile station has not sent a training waveform for 200 slots. The symbol spaces used for sending a training waveform are sent during a training period 58. In practice, training waveforms are sent less than 1% of the time. When training waveforms are not sent, pseudotraining waveforms, which serve no purpose other than that of filler data that may be detected as the pseudotraining waveform, may be sent during the training period 58.
The symbol 44 represents a synchronization (sync) symbol that a base station uses to adjust the phase, amplitude and timing based on the sync symbols received from a mobile station. The symbol 46 represents a data symbol in which the mobile station may transmit data representative of voice, audio or any other suitable information. The symbol 48 represents pilot symbols that the mobile station may transmit to the base station for the base station to use in fading detection and compensation.
As shown in the constellation of FIG. 2A, each symbol may represent four data bits that are transmitted using a quadrature amplitude modulation (QAM) scheme or any other suitable modulation scheme, such as, for example, phase shift keying (PSK), differential quadrature phase shift keying (DQPSK) or the like.
The mobile station transmits the color designators in each slot 40 it sends to the base station. For example, the mobile station typically sends the color designators in symbols 59a and 60a, as well as in symbols 59b and 60b. 
Despite the added cross talk protection provided by the 16 color designations, as TDMA base stations, such as, for example, the iDEN base stations, become more densely geographically located, cross talk may still be an issue. This is because first and second base stations having identical colors and receive frequencies may be located proximately enough for a mobile station communicating with the first base station to interfere with the second base station.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding the disclosure.